The present invention provides a mould mat and method for producing bone cement pellets, i.e. hardened pellets of a bioabsorbable bone substitute material suitable for the treatment of bone disorders and filling of bony voids or defects of the skeletal system. In particular, although not exclusively, the present invention provides a convenient and simple means to produce moulded, hardened pellets (e.g. cylindrical pellets) of a calcium-based bone cement having a range of sizes for the treatment of bone disorders and filling bone defects, with minimal bone cement wastage.
The use of calcium salt based bone substitute materials is common practice in orthopaedic surgical procedures for the filling of bony voids or gaps of the skeletal system caused by trauma, disease or surgery. These materials when implanted are intended to resorb and be replaced by new bone as they do so. They can consist of a range of different calcium based salts incorporating sulphate and/or phosphate based anions. They may be provided in a range of physical forms including granules and pellets.
There are a number of calcium based bone cement materials which may be used as bone void fillers. These are typically supplied in the form of a powder which when mixed with an aqueous liquid component forms a hardenable cohesive mass that cures and finally sets to form a calcium based bone cement.
It is to be understood that a bone cement is a bone graft or bone substitute material that hydrates, hardens and sets when powder and aqueous liquid components are mixed together. Any bone cement described herein may be used to produce bone cement pellets as disclosed.
Herein, ‘bone graft’ and ‘bone substitute’ are terms that are used interchangeably.
Calcium sulphate hemi-hydrate is one such calcium based bone substitute material. When the calcium sulphate hemi-hydrate powder is blended with an appropriate quantity of water or salt solution, the mixture hydrates to form a cohesive mass and sets with a mildly exothermic reaction to give calcium sulphate di-hydrate (i.e. the bone cement) according to the following reaction:CaSO4½H2O(Plaster of Paris)+1½H2O=CaSO42H2O(gypsum)
The calcium sulphate hemi-hydrate may be used alone or it may be used in combination with calcium phosphate based bone substitute materials including tricalcium phosphate and hydroxyapatite.
Calcium phosphate cement (CPC) is a synthetic bone graft material that was invented in 1986 by L. C. Chow and W. E. Brown, scientists at the American Dental Association. The cement is formed from a calcium phosphate based bone substitute material comprising a white powder consisting of equimolar amounts of ground Ca4 (PO4)2O (tetracalcium phosphate, TTCP) and CaHPO4 (dicalcium phosphate anhydrous, DCPA). The powder when mixed with water forms a workable paste which can be shaped during surgery to fit the contours of a wound. The paste hardens (i.e. fully sets) within 20 min to form the calcium phosphate cement, thereby allowing rapid closure of the wound. The hardening reaction, which forms nanocrystalline hydroxyapatite (HA) as the product, is isothermic and occurs at physiologic pH so tissue damage does not occur during the setting reaction.
There are now available a number of different formulations of CPCs. These may contain a range of calcium based salts including monocalcium phosphate, dicalcium phosphate, tetracalcium phosphate, octacalcium phosphate and calcium carbonate and combinations thereof. The calcium based salts are mixed together with an aqueous mixing liquid which may also contain soluble phosphates such as sodium phosphate or phosphoric acid.
Depending upon the surgical procedure being undertaken and the surgeon's preference, it may be required to use the bone cement in the form of pellets or granules. A granular bed of bone graft material contains inter-granule porosity, and this is often considered to be a prerequisite for bony in-growth. The packing density and inter-granule pore size will depend upon the pellet/granule size and size range. A granular or pelletised form of bone graft material also enables the material to be easily mixed with morselised autograft, an often conducted practice in many surgical procedures. The use of moulded pellets having a uniform pellet shape and/or size helps ensure a more predictable resorption profile.
Frequently, therapeutically active compounds are added to the hardenable bone cement which is used as a carrier for the local delivery of the therapeutically active compound. The addition to bone cement and subsequent formation into pellets represents a convenient and effective means to deliver the therapeutically active compound to the site and helps ensure a more consistent and predictable release profile of said compound to the surrounding tissue.
To enable a bone cement to be presented to the surgical site in the form of pellets, some form of pellet mould is required to aid in the formation of the pellets. A mould mat containing a plurality of cavities therein, in which the pellets are shaped, is typically used. This is typically provided as part of a kit of parts for forming the hardenable bone cement pellets. The kit of parts usually contains a powdered bone cement material, a mixing aqueous liquid, a mixing bowl, a mixing spatula, a mould mat and a scraper.
The mould mat typically takes the form of a flat mat made from a flexible rubber or polymeric material, and containing a plurality of cylindrical or hemispherical shaped cavities on one side. The bone cement is prepared in the normal manner by using a small mixing spatula or similar and mixing together all of the powder components with all of the liquid components in a bowl, pot or dish. The resulting intermediate paste (i.e. hardenable bone cement) is then pressed (i.e. pasted) into the pellet cavities, for example by use of a small mixing spatula, where it is allowed to harden and set. Once fully hardened the mould mat can be flexed to extract all pellets from the mould.
Pasting the cement into the cavities of a mould mat of the prior art can be a messy and time consuming procedure, particularly when using the small mixing spatula for this purpose. Additionally, mould mats of the prior art often contain an array of cavities whereby the distance between adjacent cavities is large relative to the cavity diameter, resulting in a large area of the surface of the mat that does not contain any cavities. The cavities are often arranged in rows and columns at right angles to one another. Within the array, the spacing between adjacent cavities within the rows and/or columns can be relatively large compared to the pellet diameter, particularly for smaller diameter pellets. This will mean that there is a relatively larger proportion of the area of the mat, within the array, that does not contain cavities. This has the effect that when bone cement is drawn down the length of the surface of the mat it encounters proportionately more mat-surface (no cavities) than cavity. This, typically, also results in a waste of expensive bone substitute material as it is difficult to ensure that all of the hardenable bone cement is used to fill all of the mould cavities. The use of the small mixing spatula to paste all of the hardening cement into the mould cavities can take longer than the setting time of the cement such that the cement hardens and sets before it is all used to fill the mould cavities. The extraction of the bone pellets through flexing of the mat can be difficult as the mat may not flex very well, or the pellets may become stuck within the mat.
The surgeon therefore is faced with the possibility of having insufficient pellets for the surgical site due to the surgeon being unable to extract some of the pellets from the mould mat of the prior art by flexing it. Those that have been extracted may have rough edges due to the bone cement residue that has been left on the mould mat surface and has hardened.
In order to provide the surgeon with the option to produce a range of pellet sizes, or shapes, and also additional quantity of pellets the mould mat may be excessively large or indeed two mould mats having cavities of differing shapes or sizes may be required. Additional mould mats add to both cost and packaging requirements which can further impact on sterilization issues and storage volumes. There is also an increase of bone cement wastage where bone cement has remained within the area between the cavities on the surface of the mould mat.
The present invention seeks to overcome one or more of the aforementioned technical problems for forming bone cement pellets.